J. Newman et al, J. Electrochem. Soc., 128, PP510-517(1971), xe2x80x9cDesalting by Means of Porous Carbon Electrodesxe2x80x9d
I. Parikhi; P. Cuatrecasas, CandEN, Aug., 26, 1985, PP17-32(1985), xe2x80x9cAffinity Chromatographyxe2x80x9d
1. Field of Invention
This invention relates generally to capacitive deionization (CDI) of liquids containing charged species, including aqueous, inorganic and organic solutions. More particularly, this invention relates to recurrent electrosorption of ions (deionization) and regeneration of electrodes whereby energy is extracted and stored in supercapacitors, ultracapacitors, or electric double layer capacitors. The present invention provides deionizors wherein purified liquids and electricity are co-generated.
2. Description of Related Art
Energy and water are two essential ingredients of modern life. Since the fossil fuel is diminishing and generates pollution at power generation, people become more eager in searching for alternative sources of energy. Therefore, renewable energy sources such as solar power, wind power, wave power, and geothermal heat have been explored and commercialized. Many international automakers are aggressively developing fuel cells for pollution-free electric vehicles. All of the above endeavors are aimed to reduce CO2 emission and to use natural free resources such as sun and water for energy production. Production of energy is no easy matter, hence conservation of energy that includes controlled usage and responsive extraction of energy deserves attention. There are numerous viable ways for retrieving residual energy that would be otherwise wasted. For example, U.S. Pat. No. 6,326,763 issued to King et al disclosed a regenerative braking system that can store electricity converted from the remaining momentum of vehicle during periods of deceleration due to braking for stop or moving down hill. Ultracapacitors were proposed in ""763 to extract the residual energy that is generally dissipated as heat. In another example, U.S. Pat. No. 6,267,045 issued to Wiedemann et al revealed a cooking device containing an energy storage and energy extraction system wherein energy is exchanged in the form of latent heat.
Less than 1% of water on the earth surface is suitable for direct use. Fresh water will be one of the precious commodities in the 21st century. In lieu of rainfall, desalination of seawater is probably the most plausible means to attain fresh water. Among the commercial desalination methods, distillation dominates the market with 56% share, reverse osmosis (RO) possesses 40%, while freezing and electrodialysis seize the rest. The aforementioned methods though are different in the purification mechanisms, they are all utilized to reduce the total dissolved solids (TDS) which is a measure of charged species in solutions so that seawater can become potable. Reduction of TDS, or deionization, is also an ultimate goal for waste liquid treatments where ion exchange and RO are most frequently used. In purifying seawater or waste liquids, the employed method should be a low energy-consumption, pollution-free, and long service-life technique. In fulfilling the foregoing requirements, capacitive deionization (CDI) is a superior method than ion exchange, RO, and other techniques for deionization. There are five reasons to vindicate the supreme merit of CDI: (1) CDI uses a DC electric field for adsorbing and removing ions from solutions, the process is quick and controllable with minimal energy consumption. (2) Energy that is input for electrosorption can be extracted and stored for latter use or other applications. No energy recovery is available in any of the aforementioned separation methods. (3) While energy is transferred from the CDI electrodes to a load, the electrodes are restored simultaneously. Regeneration of CDI electrodes by energy extraction is prompt without using chemicals and without producing pollution. (4) CDI can directly deionize seawater or solutions with TDS higher than 35,000 ppm. Deionization and regeneration can be repeated numerous times until the liquids are clean, and the electrodes are not degraded by the high salt content. Whereas RO, electrodialysis, and ion exchange are better utilized for treating low salt-content solutions. Otherwise, their expensive membranes or resins will be damaged quickly. (5) Ions that are adsorbed by the CDI electrodes can be discharged in a concentration reservoir for recycling useful resources or for sludge disposal. Extraction of ions by CDI is a non-destructive process, thus some ions may be processed for reuse. The invention will demonstrate all of the foregoing five unique features of the CDI technique in the latter section of detailed description. Incidentally, energy recovery in the deionizers is consistent with the ultimate principle of free energy tapping, that is, no fuel should be added and no pollutant should be emitted.
CDI is a separation methodology that is known for more than 40 years (J. Newman et al, 1971). Just to name a few, U.S. Pat. Nos. 5,799,891, 5,858,199, 5,954,937 and 6,309,532 are all intended to commercialize the CDI technique. Particularly, ""532 issued to Tran et al disclosed the use of electrical discharge for regenerating electrodes. Rather than reclaiming the residual energy, the electricity is dissipated by shorting or reverse polarity (claims 4, 18, 21, and 23). Shorting a fully charged capacitor may cause electrical hazards particularly when the energy accumulated is immense. It is known to people skilled in the art that reverse polarity would momentarily expel the adsorbed ions from the electrodes. However, the ions would leave one electrode and then be adsorbed by the other electrode, unless an alternate polarity reversal of appropriate frequency is applied in conjunction with a large quantity of fluid for flushing the desorbed ions out of the cell. This may explain why 40 liters of liquid was used for every cycle of regeneration in Example 1 of ""532. In addition, one cycle regeneration of the CDI electrodes of ""532 takes several hours to accomplish, such lengthy process is unprofitable for commercial application. As indicated by FIG. 12 of ""532, deionization as proposed was equally slow as a reduction of TDS by 59 ppm (100 xcexcS divided by conversion factor 1.7) using 150 pairs of 10xc3x9720 cm2 electrodes, or a total geometric area of 30,000 cm2, took 10 minutes of processing time. Moreover, ""532 taught a serpentine liquid flow pattern in a complex cell as shown by FIG. 3 wherein 150 pairs or other combination of electrodes were stacked and compressed. For creating the liquid path, apertures were specifically fabricated on the electrode supports whereon carbon aerogel, lithographically perforated metal, or costly metal carbides were used as the electrosorptive medium. Usage of the foregoing designs and materials will add cost to the CDI cells and present difficulties to operation, as well as maintenance. In comparison, disclosures of the present invention will furnish cost-effective, high-throughput, and user-friendly deionizers for purifying contaminated liquids and for desalinating seawater. After partial or complete adsorption of ions, the deionizers can be discharged at different rates to deliver constant currents or peak currents to different loads as required. In other words, the deionizers can be utilized as liquid purifiers, as energy-storage devices, and as power converters.
Practically, CDI has adopted the charging mechanism of supercapacitor (other nomenclatures for the device include ultracapacitor and electric double layer capacitor) for removing ions from solutions in this invention. Supercapacitor is an electrochemical capacitor that can store static charges up to several thousands of farad (F), and it can be charged and discharged quickly. As the electrodes of supercapacitor accumulate ions on their surface, a DC potential is developed with increasing charges across the positive and negative electrodes of the capacitor. Such voltage rise with ion accumulation relationship is also observed during deionization by CDI process. Therefore, voltage can be used to determine if the CDI electrodes have reached an adsorption maximum, or they have reached an equilibrium state where the induced potential is equal to the applied voltage. In either case, the CDI electrodes require regeneration for further service. Following the general electrode configurations of supercapacitors, that is, stacking or winding, the CDI electrodes are similarly constructed and assembled into modules but with two variations. Firstly, unlike the separators reserve electrolytes for the supercapacitors, components other than the electrosorptive medium in the CDI modules should neither adsorb nor retain ions. Secondly, unlike the electrodes of supercapacitors are enclosed in protecting housings, the CDI electrodes are merely secured by simple means such as tape without encapsulation. Hence, the CDI electrodes of the present invention are widely open to the surroundings, and fluids to be treated have free access to the electrodes. With the foregoing flow-through design, the CDI modules can be placed in fluid conduits for deionization, or they can be submerged in liquids and cruised like a submarine to remove ions.
Not only the electrodes are assembled using the minimal amount of supporting materials, readily available activated carbon of low price is also used as the electrosorptive medium for further reducing the cost of CDI modules. Carbon material is deposited onto an electrically conductive substrate by an inexpensive process such as roller coating to form the CDI electrodes. With cost-effective materials, easy fabrication of electrodes, and simple assembly of electrodes, the deionizers can become reliable consumer products affordable to families and industries.
Just like the stored energy of supercapacitors can be quickly extracted via discharge, the residual energy of the CDI electrodes after electrosorption of ions is also available for fast tapping. Though the energy that is reclaimed is far less than the energy that is input for deionization, the residual energy is free and addible for practical applications. Besides, same as the supercapacitors having 100% discharge depth, the energy stored on the CDI electrodes can be completely drained as well, and the electrodes are thoroughly cleaned as a consequence of energy recovery. To store the residual energy reclaimed from the CDI modules, supercapacitors, or ultracapacitors, or electric double layer capacitors are particularly well suited as the storage devices. This is due to the devices are more efficient in storing energy than other devices such as batteries and flywheels. As long as the source voltage is higher than the voltage of supercapacitors, the capacitors can always be charged regardless of the magnitude of charging current. When the CDI modules are installed on a carousel or Ferris wheel, the electrodes can then be reciprocally and continuously engaged in deionization and regeneration. Because of swiftly recurrent deionization and regeneration, the deionizers have high throughputs for purifying contaminated liquids as well as for desalinating seawater. It is experimentally observed that the repeated deionization and regeneration cause no damage to the deionizers.
Restoration of the CDI electrodes by energy extraction is operational in any liquids including seawater. Only the adsorbed ions are discharged to a liquid, thus the liquid has no influence on regeneration and there is no second pollution. Furthermore, no flushing liquid or regenerant fluid is required to discharge the ions for they are automatically dissipated at energy recovery. Except a minimal amount of clean liquid may be needed to rinse the electrodes, the regeneration produces no waste liquid. Ions are adsorbed by the CDI electrodes under a DC electric field whereby the applied voltage can be controlled below the decomposition potentials of ions. Thence, the CDI electrodes may be utilized as a magnet to non-destructively take ions out of liquids and to place them in a concentrating container. Once the ions are concentrated in a small volume of liquid, useful resources can be easily recycled or the sludge can be effectively disposed. It is during the period when the restored electrodes are returned from the concentrating container to the deionization chamber that rinsing may be required.
Deionization of solutions by CDI only requires the application of low DC voltages, thus it is operable by batteries, fuel cells, and solar cells. Most of the latter devices have poor power densities. Nevertheless, after the residual energy is stored in supercapacitors, the capacitors can then deliver peak powers to various heavy loads. From this aspect, the deionizers behave as power converters using adsorption and desorption for energy transference. Because of various electrical resistances and other forms of energy loss such as electrolysis, the cycle of adsorption and desorption, or charge and discharge is not a perpetual motion. Nevertheless, using the deionizer of this invention as a power converter may provide some practical applications.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.